Aprii. 19, 1907] 



SCIENCE 



617 



covering that part of the slit through which 

 the light of the umbra passes. It is ob- 

 vious that a large image of the sun will be 

 required in this work. 



The spectroheliograph can be applied to 

 other studies of absorption. The H^ fioc- 

 culi, for example, are reduced in bright- 

 ness near the sun's limb much more than 

 the E2 flocculi, presumably because the 

 latter lie at a higher level. These differ- 

 ences can be studied photometrically on 

 spectroheliograph plates made for the pur- 

 pose. Since it is a question just what level 

 is represented by the background (between 

 the flocculi) in calcium, hydrogen or iron 

 photographs, the instrument should be ar- 

 ranged so as to permit photometric com- 

 parisons with the light of the photosphere, 

 of practically the same wave-length as the 

 calcium, hydrogen or iron line employed. 



These new applications of the spectro- 

 heliograph have only recently occurred to 

 me, and are mentioned because of their 

 suitability for use with instruments con- 

 taining prisms of ordinary height, capable 

 of photographing only narrow zones of the 

 solar image. Numerous other problems 

 might be mentioned, such as the compara- 

 tive study of Hi, H^ and H^ photographs, 

 and of calcium, hydrogen, and iron images ; 

 the distribution of the flocculi in latitude 

 and longitude, their varying area, as bear- 

 ing on the solar activity and on terrestrial 

 phenomena, and their motion in longitude, 

 as measuring the solar rotation. But limi- 

 tations of time forbid more than a mere 

 reference to work and methods, the details 

 of which are discussed elsewhere. My pur- 

 pose has been accomplished if I have shown 

 that with comparatively simple instru- 

 mental means any careful observer may 

 secure important results. In much of this 

 work it is desirable that investigators oc- 

 cupied with similar problems should co- 

 operate with one another. The Inter- 

 national Union for Cooperation in Solar 



Research was organized with this end in 

 view. It has already inaugurated solar 

 studies, on a common plan, in several dif- 

 ferent fields, and is preparing to extend 

 the range of its activities in the near 

 future. Harold Jacoby, 



For the Council 



SCIENTIFIC BOOKS 

 Roch Minerals, their Chemical and Physical 



Characters and their Determination in Thin 



Sections: Joseph P. Iddings. Wiley and 



Sons, New York. Pp. xii + 548, with 



numerous figures in text. 



In presenting this work the author and pub- 

 lishers have won the gratitude of every Amer- 

 ican student in petrography, and of every 

 teacher as well. Heretofore, the only sys- 

 tematic and comprehensive treatises available 

 have been the ' Mikroskopische Physiographie ' 

 of Professor Eosenbusch or Professor Id- 

 dings's condensed translation of the same, and 

 the works of Fouque and Levy. While no one 

 would for a moment wish to disparage the 

 work of one who has fairly earned the title 

 of father of modern petrography, yet, as may 

 readily be comprehended, the manner of pre- 

 sentation in the first-named publication, as 

 well as the language in which it is presented, 

 is German, and not always easy of compre- 

 hension to the average American student. 



There have been, it is trtie, other works on 

 the subject, in English, as Professor Luquers's 

 ' Minerals in Thin Sections ' and Harker's 

 ' Petrology for Students,' but such make no 

 pretense at completeness, and it has remained 

 for Professor Iddings to give us a book as 

 comprehensive and systematic as those of the 

 German and French writers noted. 



Within the limits of some 550 pages Pro- 

 fessor Iddings includes not merely a descrip- 

 tion of the optical properties and methods of 

 determination of all the ordinary rock-form- 

 ing minerals, but also chapters on their 

 chemical and physical characters. The crit- 

 ical chapter of the book is undoubtedly that 

 relating to the optical properties of minerals, 

 and it is apparently in recognition of this that 

 the author has devoted upwards of 100 pages 



